Herzog has faced a barrage of criticism over his attempts to negotiate for his party to join Israeli Prime Minister Binyamin Netanyahu's coalition and over Labor's slide in opinion polls.

The country's last Labor prime minister was Ehud Barak from 1999 to 2001.

Ahead of the 2015 elections, Labor joined forces with Tzipi Livni's Hatnua to form the Zionist Union, which won 24 seats in the 120-seat parliament, becoming the largest opposition to Netanyahu's government.

Supporters of the two candidates in the runoff hope their backgrounds will widen Labor's support.

Both are of Moroccan descent, rare for a leader of a major party in Israel, and may be able to increase support for Labor among Mizrahi Jews -- those of Middle Eastern or North African origin.

They also both support a two-state solution to the Israeli-Palestinian conflict.

But beyond that, Peretz and Gabai have little in common.

Peretz, 65, is a veteran politician, former trade union leader and ex-defense minister.

He was previously head of Labor from 2005-2007 and is the longest serving member of the Knesset, or parliament.

His first stint as leader ended after Israel lost the Second Lebanon war during Peretz's tenure as defense minister.

Peretz has received backing from Herzog and other party insiders in the runoff.

Gabai, 50, is seen as a fresh face, having formerly headed Israeli telecommunications firm Bezeq before joining politics. He has never been a member of Israel's parliament.

In 2014, he joined forces with former Likud minister Moshe Kahlon to form center-right Kulanu.

Kulanu won 10 seats in the 2015 general elections and joined Netanyahu's coalition, with Gabai appointed environment minister despite not being a parliament member.